1. Field of the Invention
The present invention relates to a method for controlling an image forming apparatus that forms an electrostatic latent image by irradiating the charged surface of a photosensitive body with a laser beam to form an image.
2. Description of the Related Art
An electrophotographic image forming apparatus includes a charging unit that uniformly charges the photosensitive surface of an image bearing member (for example, a photosensitive drum), a latent-image forming unit that forms an electrostatic latent image corresponding to image information on the charged photosensitive surface, and a developing unit that develops the electrostatic latent image. The electrophotographic image forming apparatus further includes a transfer unit that transfers the electrostatic latent image developed with toner to a sheet of recording paper and successively forms images while rotating the photosensitive surface of the photosensitive drum.
In such an image forming apparatus, a change in image density, a change in tone reproduction, and the like occur under the influence of a short-term change due to a change in the environment in which the apparatus is placed, a change in the environment in the apparatus, and the like, and a long-term change due to a change (deterioration) of the photosensitive drum or toner over time. That is to say, in order to unify the density, tone reproduction, and the like of an output image, correction needs to be performed in view of these changes.
In view of these problems, a method is disclosed in Japanese Patent Laid-Open No. 7-264427 for effectively utilizing the maximum density that can be expressed in consideration of a deterioration of the maximum image density. Specifically, after the condition for forming an image is adjusted so as to be higher than a target maximum density, the transfer characteristic of a transformation unit that performs density transformation of input image data is adjusted. The following method exists for controlling the stability of densities in a high density range. A desired maximum density is obtained by obtaining a target value of the potential of the surface of a photosensitive drum on the basis of the correlation between contrast potentials and the densities of the maximum density patches of individual colors of yellow (Y), magenta (M), cyan (C), and black (Bk) and determining the charging bias and the developing bias.
Moreover, a technique is disclosed in Japanese Patent Laid-Open No. 10-239924. In this technique, for each of at least two combinations of charging biases and developing biases, a reference patch image that is generated under the same exposure conditions is formed on an image bearing member or another image medium while changing both of the charging bias and the developing bias at the same time. Then, the reference patch image is read, and the settings of the charging bias and the developing bias are determined on the basis of the read data.
In the method disclosed in Japanese Patent Laid-Open No. 7-264427, the stability of densities in a high density range is considered. However, since the charging bias and the developing bias are determined from the density of one patch, it is hard to perform precise correction.
Moreover, in the technique disclosed in Japanese Patent Laid-Open No. 10-239924, precise control can be performed by obtaining the correlation between the reference patch and the density while changing the charging bias and the developing bias. However, it takes long time to perform adjustment.